Transgenic Environmental Biosensors

If transgenic fish can be engineered to serve as living sentries for aquatic mutagens and other pollutants, it should come as no surprise that genetic engineers plan to create and deploy terrestrial creatures in similar fashion. In the last century, miners often placed caged canaries in working tunnels to serve as early-warning alarms to impure air. Likewise, today's biotechnologists hope to deploy a wide variety of transgenic plants, animals, and microbes as sensitive biodetectors of cryptic and often hazardous foreign substances in the environment. This essay touches upon several examples under current research and development.

Sedentary plants, being closely tied to the land, should be favorable organisms for monitoring toxic soil chemicals, of which, unfortunately, there are many. Particularly since the Industrial Revolution, humans have burdened natural soils around the world with myriad pollutants ranging from complex organic poisons such as PCBs (polychlorinated biphenyls, used in pesticides), to simple inorganic metals that can be highly toxic in high doses, to radioactive elements. Now, biotechnologists are attempting to engineer plants to help detect and report such soil contaminants to humans.

Transgenic mustard plants (Arabidopsis) recently were designed to assay for heavy metals. Annually, humans dump more than 1.3 million tons of zinc, 950,000 tons of copper, 790,000 tons of lead, and 20,000 tons of highly toxic cadmium into the environment, for example. Much of this inorganic material contaminates soils near mines and various industrial sites. Some of the heavy metals, such as zinc and copper, are essential trace elements, necessary for life in miniscule quantities, and accordingly are added to many vitamin pills and processed foods. However, these and other metals become health risks at the much higher doses that characterize many polluted areas. When transferred to living tissues, heavy metals at elevated concentrations often exert their toxic effects by inducing damage to DNA molecules and cells and by promoting cancer, probably via their mutagenic actions.

The experimental mustard plants were engineered in such a way as to record mutations induced by heavy metals or other mutagens that might be present in soils. By inserting a transgene (for the enzyme ^-glucuronidase) that permits a simple histochemical assay, new mutations could be detected and quantified in the plants' tissues. The researchers found that transgenic mustards exposed to increasing concentrations of heavy metals did indeed show significantly increased mutation frequencies, making the GM plants potentially useful as living biomonitors of soil pollution.

Transgenic mustard plants have also been used to scan for radioactive mutagens. On April 26, 1986, a meltdown at the Chernobyl nuclear reactor in the Soviet Union resulted in the release of a radioactive cloud that blanketed more than 600 km2 of surrounding land with cesium-137, strontium-90, americium-241, and other radionuclides. Genetic studies of humans and indigenous animals near Chernobyl have indicated increased rates of germline mutations from the ionizing radiations, but plants would obviously be more suitable and convenient subjects for directed experimental analyses of the situation. Accordingly, researchers recently used GM mustard plants to assess the mutational potential latent in assorted Chernobyl soils.

Such living sensors of mutagenic agents in the environment have the advantage of revealing bioavailable levels of pollution relevant to plant or animal health. However, other transgenic detection systems may be far simpler and more straightforward. In Aberdeen, Scotland, a startup company, Remedios, has developed GM bacteria that glow yellow in clean soils, but not when the soils are contaminated by particular chemicals. By using a battery of pollution-sensitive microbes carrying luminescent transgenes derived from fireflies or marine bacteria, the company can screen for volatile organic solvents like benzene, toluene, and xylene, for toxic nonvolatile organic compounds like PCBs, and for toxic inorganic substances like heavy metals.

Another potential use of transgenic microbes is in landmine detection. Some bacteria can sense TNT (trinitrotoluene) or other explosive chemicals, and scientists would like to capitalize on this inherent ability to help find thousands of hidden landmines that cause horrific carnage in former war-torn regions of the world. The idea is to fuse a fluorescence transgene to a bacterial TNT-sensing gene and then spray (e.g., by crop-duster planes) the GM microbes over suspected minefields. Landmines leak small amounts of TNT over time. Thus, in any transgenic bacteria that fall near a mine, both the TNT-sensing gene and its adjacent reporter transgene become activated. By then shining an ultraviolet light over the affected landscape at night, the precise location of each landmine is revealed as a fluorescent spot in the soil. In experiments with this approach, transgenic bacteria located all five buried sources of TNT in a 300-meter test plot in South Carolina.

Sometimes, fluorescent reporter genes are engineered into creatures for the purpose of identifying the transgenic organisms themselves, rather than abiotic substances in their environment. One example involves cotton bollworms. As described earlier (see chapter 4), one traditional method of controlling this insect pest is to release large numbers of laboratory-sterilized individuals into nature to compete with fertile native moths for mates. Once released, however, the sterile individuals normally cannot be distinguished from the wild animals by simple visual inspection. So, using recombinant DNA techniques, researchers added a luminescent transgene to some lab-reared batches of sterile bollworms. These animals fluoresce under UV light, a property that could assist in future efforts to monitor the abundance and distribution of released specimens.

The fluorescent GM organisms and their applications presented thus far may seem bizarre, but one final example is genuinely otherworldly. Scientists recently announced plans to use tiny transgenic plants to monitor environmental conditions on Mars. The National Aeronautics and Space Administration has scheduled a 2007 mission to explore Mars, and onboard the spaceship may well be mustard seeds engineered with reporter genes designed to incandesce if particular substances are present. After their 286-million-mile journey, the seeds would be planted in Martian dirt (and presumably watered, fertilized, and tended) by a gardener robot, and a camera would record the outcomes. Any germinated plants exposed to heavy metals would turn fluorescent green, for example, and those exposed to peroxides would turn blue. Such experiments might add a bit more color to Mars, as well as cast new light upon its surface conditions.

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